Coated optical fibers have various structures which are roughly classified, for example, into (1) a three-layer structure, (2) a loose structure, (3) an FRP structure, and (4) a tape structure.
The coated optical fiber of the three-layer structure is obtained by coating a varnish such as a silicone varnish, on an optical fiber having a diameter of 100 to 150 microns, forming a cushion layer of a silicon rubber having a thickness of 100 to 150 microns thereon as a primary coating, and further providing a secondary coating of a thermoplastic resin such as nylon-12, and has an outside diameter of about 1 mm.
However, since nylon-12 attains a temperature of 200.degree. C. or higher during molding, the optical fiber may be deteriorated. Furthermore, because the resin is crystalline, it shrinks greatly during or after cooling and solidification and strains occur as microbending in the optical fiber to degrade its transmission characteristics.
The FRP coated optical fiber is obtained by using glass fibers as a coating on an optical fiber and solidifying the glass fibers by a thermosetting resin. Since the FRP coated optical fiber can be formed at low temperatures, there is no likelihood of deteriorating the optical fiber during the coating step. It is difficult, however, to procure a resin having both excellent rigidity and excellent strength against fracture by bending, and moreover, the FRP coated optical fiber has a low strength against an external impact.
The coated optical fiber of the tape structure is obtained by holding an optical fiber coated with a curable resin between coating films, and then bonding them by heating or by using an adhesive. When the bonding is effected by heating, the shrinkage of the resin becomes a problem. When an adhesive is used, a drying step is required, and this reduces the productivity.
Resins which can be used for coating optical fibers must have a high modulus of elasticity, excellent impact strength, resistance to cracking against bending stress, a low shrinkage after melt-shaping, moldability at low temperatures, a reduced molding strain even upon high-speed extrusion and good heat aging resistance. No resin meeting all of these requirements is presently available, and nylon-12 is employed although it gives rise to some problem in regard to strains during molding or moldability at low temperatures.
As a result of extensive investigations in order to provide a resin which meets these requirements, it has been found that a ring-opened polymer or ring-opened copolymer of a norbornene derivative having a polar substituent is suitable for coating optical fibers which require thermoplasticity due to a high modulus of elasticity, excellent impact strength, substantially low molding strains (since it is an amorphous resin), and low-temperature moldability.